This invention relates to the desulfurization of coal or coal water mixtures employing titanium compounds. Applicants have found that the treatment of some coals with an aqueous solution of titanous chloride (titanium trichloride and TiCl.sub.4 are equivalent representations of this material) results in the liberation of copious amounts of hydrogen sulfide. This result is surprising in light of the difficulty generally encountered in the removal of coal-bound sulfur.
It has long been known that heating coal to coking temperatures will liberate much of the contained sulfur. U.S. Pat. No. 3,166,483 describes a process whereby coal is partially desulfurized with molten KOH/NaOH at temperatures between 200.degree. and 400.degree. C. Alternatively, U.S. Pat. No. 3,567,377 teaches that less corrosive Na.sub.2 CO.sub.3 may be employed but temperatures in the range of 850.degree.-1100.degree. C. are required. A significant body of work has been directed toward employing air in oxidative desulfurization but generally high temperatures and pressures are employed, as shown in U.S. Pat. No. 3,824,084. In these prior art processes, fuel losses generally occur and the oxidized sulfur is frequently difficult to remove.
U.S. Pat. No. 1,643,272 describes a process for refining sulfur-containing mineral oils by treatment with titanium tetrachloride. This process requires dilution with an inert substance such as clay, bleaching earth, Fuller's earth, or the like in order to improve the utilization of the titanium tetrachloride reagent and to improve the color of the recovered oil. Obviously the reference process is an anhydrous process where the titanium tetrachloride and the inert substance, Fuller's earth for example, are added to the mineral oil. The presence of water in this process would render the process inoperative since the oil/water phases would not mix. In addition titanium tetrachloride is unstable in the presence of water. In contrast, in the present invention the coal is desulfurized by treatment with an aqueous solution of TiCl.sub.3. This solution is active toward coal in water and is effective at room temperature in the absence of any bleaching additive to provide desulfurization by H.sub.2 S liberation in the aqueous phase.
The instant invention describes a reductive process, with distinct advantages over the prior processes, that will accomplish desulfurization and will not result in coal loss. Further, hydrogen sulfide is a gas of only moderate water solubility, allowing for reasonable sulfur separation schemes. Titanous chloride is the preferred titanium compound for use in the present invention, although other tervalent water stable titanium salts may be employed. The TiCl.sub.3 has two distinct advantages; it is effective at room temperature and can also be used in aqueous solutions. Most other reagents that have been tested for removing coal-bound sulfur are rapidly destroyed by water.
The reaction of the present invention is as follows: ##STR1## The reaction is conveniently carried out at ambient pressure and temperatures as low as room temperature are effective. TiCl.sub.3 is commercially available in solid pure form or in hydrated form as titanium trichloride hexahydrate, the latter used commercially as a bleaching or stripping agent in the dyeing industry. It is also conveniently available as an aqueous solution.
When a solution of titanous chloride is reacted with iron disulfide (pyrite) in the presence of water, a vigorous evolution of hydrogen sulfide results. However, this reagent is inactive toward dibenzothiophene. From these results it would appear that titanous chloride is most effective with inorganic sulfur.
Those skilled in the art will readily recognize that hydrochloric acid will interact with iron sulfides to liberate hydrogen sulfide. Furthermore, some early transition metal halides partially hydrolyze to generate HCl. However, we have been unable to observe any reaction between dilute hydrochloric acid and coal, nor are we aware of any reports of such reactions occuring under conditions approximating those of the instant invention.
That the chloride anion or metal chlorides in general were ineffective was demonstrated by allowing coal to sit in the presence of aqueous solutions of sodium chloride, manganese chloride, cobalt chloride, stannous chloride and zinc chloride. In all cases no hydrogen sulfide could be detected. In contrast, in those reactions employing titanous chloride, significant bubbling is observed with copious liberations of H.sub.2 S. Like titanous chloride, both manganese and stannous chloride can be regarded as reducing agents but they are ineffective in liberating H.sub.2 S. Zinc and stannous chlorides have been used as catalysts in treatment of coal, specifically its hydrogenation, but under much more severe conditions. The reducing properties of titanous chloride are well known, since it has been employed in titrations for nitro and azo groups as well as for the ferric ion. However, the present invention represents its first use in the desulfurization of coal.